For achieving compensation of three-phase unbalanced harmonics

one three-phase fourwire active filter is composed by three single-phase active filters in Y-connection. Two-phase current is simulated by digital phase shifting method

and the detection of harmonic current of single phase system by way of instantaneous reactive power theory is realized. The simulation model of threephase four-wire active power filter is established and application features of three-phase four-wire active power filter at different wiring modes are studied. The experiment system of the active power filter is established. It is shown by the result that the unbalanced harmonic in the system can be filtered effectively by the three-phase four-wire active filter.